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11. Clipped disks

11.1 The Linux IDE 8 GiB limit

The Linux IDE driver gets the geometry and capacity of a disk (and lots of other stuff) by using an ATA IDENTIFY request. Linux kernels older than 2.0.34/2.1.90 would not believe the returned value of lba_capacity if it was more than 10% larger than the capacity computed by C*H*S. However, by industry agreement large IDE disks (with more than 16514064 sectors) return C=16383, H=16, S=63, for a total of 16514064 sectors (7.8 GB) independent of their actual size, but give their actual size in lba_capacity.

Since versions 2.0.34/2.1.90, Linux kernels know about this and do the right thing. If you have an older Linux kernel and do not want to upgrade, and this kernel only sees 8 GiB of a much larger disk, then try changing the routine lba_capacity_is_ok in /usr/src/linux/drivers/block/ide.c into something like

static int lba_capacity_is_ok (struct hd_driveid *id) {
        id->cyls = id->lba_capacity / (id->heads * id->sectors);
        return 1;
For a more cautious patch, see 2.1.90.

11.2 BIOS complications

As just mentioned, large disks return the geometry C=16383, H=16, S=63 independent of the actual size, while the actual size is returned in the value of LBAcapacity. Some BIOSes do not recognize this, and translate this 16383/16/63 into something with fewer cylinders and more heads, for example 1024/255/63 or 1027/255/63. So, the kernel must not only recognize the single geometry 16383/16/63, but also all BIOS-mangled versions of it. Since 2.2.2 this is done correctly (by taking the BIOS idea of H and S, and computing C = capacity/(H*S)). Usually this problem is solved by setting the disk to Normal in the BIOS setup (or, even better, to None, not mentioning it at all to the BIOS). If that is impossible because you have to boot from it or use it also with DOS/Windows, and upgrading to 2.2.2 or later is not an option, use kernel boot parameters.

If a BIOS reports 16320/16/63, then this is usually done in order to get 1024/255/63 after translation.

There is an additional problem here. If the disk was partitioned using a geometry translation, then the kernel may at boot time see this geometry used in the partition table, and report hda: [PTBL] [1027/255/63]. This is bad, because now the disk is only 8.4 GB. This was fixed in 2.3.21. Again, kernel boot parameters will help.

11.3 Jumpers that select the number of heads

Many disks have jumpers that allow you to choose between a 15-head an a 16-head geometry. The default settings will give you a 16-head disk. Sometimes both geometries address the same number of sectors, sometimes the 15-head version is smaller. There may be a good reason for this setup: Petri Kaukasoina writes: `A 10.1 Gig IBM Deskstar 16 GP (model IBM-DTTA-351010) was jumpered for 16 heads as default but this old PC (with AMI BIOS) didn't boot and I had to jumper it for 15 heads. hdparm -i tells RawCHS=16383/15/63 and LBAsects=19807200. I use 20960/15/63 to get the full capacity.' For the jumper settings, see

11.4 Jumpers that clip total capacity

Many disks have jumpers that allow you to make the disk appear smaller than it is. A silly thing to do, and probably no Linux user ever wants to use this, but some BIOSes crash on big disks. The usual solution is to keep the disk entirely out of the BIOS setup. But this may be feasible only if the disk is not your boot disk.

Clip to 2.1 GB

The first serious limit was the 4096 cylinder limit (that is, with 16 heads and 63 sectors/track, 2.11 GB). For example, a Fujitsu MPB3032ATU 3.24 GB disk has default geometry 6704/15/63, but can be jumpered to appear as 4092/16/63, and then reports LBAcapacity 4124736 sectors, so that the operating system cannot guess that it is larger in reality. In such a case (with a BIOS that crashes if it hears how big the disk is in reality, so that the jumper is required) one needs boot parameters to tell Linux about the size of the disk.

That is unfortunate. Most disks can be jumpered so as to appear as a 2 GB disk and then report a clipped geometry like 4092/16/63 or 4096/16/63, but still report full LBAcapacity. Such disks will work well, and use full capacity under Linux, regardless of jumper settings.

Clip to 33 GB

A more recent limit is the 33.8 GB limit. Linux kernels older than 2.2.14 / 2.3.21 need a patch to be able to cope with IDE disks larger than this.

With an old BIOS and a disk larger than 33.8 GB, the BIOS may hang, and in such cases booting may be impossible, even when the disk is removed from the CMOS settings.

Therefore, large IBM and Maxtor and Seagate disks come with a jumper that make the disk appear as a 33.8 GB disk. For example, the IBM Deskstar 37.5 GB (DPTA-353750) with 73261440 sectors (corresponding to 72680/16/63, or 4560/255/63) can be jumpered to appear as a 33.8 GB disk, and then reports geometry 16383/16/63 like any big disk, but LBAcapacity 66055248 (corresponding to 65531/16/63, or 4111/255/63). Similar things hold for recent large Maxtor disks.

Below some more details that used to be relevant but probably can be ignored now.


With the jumper present, both the geometry (16383/16/63) and the size (66055248) are conventional and give no information about the actual size. Moreover, attempts to access sector 66055248 and above yield I/O errors. However, on Maxtor drives the actual size can be found and made accessible using the READ NATIVE MAX ADDRESS and SET MAX ADDRESS commands. Presumably this is what MaxBlast/EZ-Drive does. There is a small Linux utility setmax.c that does the same. Only very few disks need it - almost always CONFIG_IDEDISK_STROKE does the trick.

For drives larger than 137 GB also READ NATIVE MAX ADDRESS returns a conventional value, namely 0xfffffff, corresponding to 137 GB. Here READ NATIVE MAX ADDRESS EXT and SET MAX ADDRESS EXT (using 48-bit addressing) are required. The setmax utility does not yet know about this. A very small patch makes 2.5.3 handle this situation.

Early large Maxtor disks have an additional detail: the J46 jumper for these 34-40 GB disks changes the geometry from 16383/16/63 to 4092/16/63 and does not change the reported LBAcapacity. This means that also with jumper present the BIOS (old Award 4.5*) will hang at boot time. For this case Maxtor provides a utility JUMPON.EXE that upgrades the firmware to make J46 behave as described above.

On recent Maxtor drives the call setmax -d 0 /dev/hdX will give you max capacity again. However, on slightly older drives a firmware bug does not allow you to use -d 0, and setmax -d 255 /dev/hdX returns you to almost full capacity. For Maxtor D540X-4K, see below.


For IBM things are worse: the jumper really clips capacity and there is no software way to get it back. The solution is not to use the jumper but use setmax -m 66055248 /dev/hdX to software-clip the disk. "How?" you say - "I cannot boot!". IBM gives the tip: If a system with Award BIOS hangs during drive detection: Reboot the system and hold the F4 key to bypass autodetection of the drive(s). If this doesn't help, find a different computer, connect the drive to it, and run setmax there. After doing this you go back to the first machine and are in the same situation as with jumpered Maxtor disks: booting works, and after getting past the BIOS either a patched kernel or a setmax -d 0 gets you full capacity.

Thomas Charbonnel reports on a different approach: "I had a 80 GB IBM IC35L080AVVA07-0 drive and installed IBM's Disk Manager. Installed my boot loader on the drive's MBR. Everything worked fine. Note that the IDE drive must become the boot drive so that one can install only one 34+ GB drive using this approach."


Seagate disks have a jumper that will clip the reported number of cylinders to 4092 on drives smaller than 33.8 GB, while it will limit the reported LBA capacity (Identify words 60/61) to 33.8 GB on larger disks.

For models ST-340810A, ST-360020A, ST-380020A: The ATA Read Native Max and Set Max commands may be used to reset the true full capacity.

For models ST-340016A, ST-340823A, ST-340824A, ST-360021A, ST-380021A: The ATA Set Features F1 sub-command will cause Identify Data words 60-61 to report the true full capacity.

Maxtor D540X-4K

The Maxtor Diamond Max drives 4K080H4, 4K060H3, 4K040H2 (aka D540X-4K) are identical to the drives 4D080H4, 4D060H3, 4D040H2 (aka D540X-4D), except that the jumper settings differ. A Maxtor FAQ specifies the Master/Slave/CableSelect settings for them, but the capacity clip jumper for the "4K" drives seems to be undocumented. Nils Ohlmeier reports that he experimentally finds that it is the J42 jumper ("reserved for factory use") closest to the power connector. (The "4D" drives use the J46 jumper, like all other Maxtor drives.)

However, it may be that this undocumented jumper acts like the IBM jumper: the machine boots correctly, but the disk has been clipped to 33 GB and setmax -d 0 does not help to get full capacity back. And the IBM solution works: do not use any disk-clipping jumpers, but first put the disk in a machine with non-broken BIOS, soft-clip it with setmax -m 66055248 /dev/hdX, then put it back in the first machine, and after booting run setmax -d 0 /dev/hdX to get full capacity again.

In the meantime, some docs and pictures have appeared on the Maxtor site, confirming part of the above. Compare

Western Digital

Some info, including the settings for capacity-clipping jumpers, is given on the Western Digital site. I do not know what precisely these jumpers do.


If an IDE/ATA disk has support for the Host Protected Area (HPA) feature set, then it is possible to set the LBA capacity to any value below the actual capacity. Access past the assigned point usually leads to I/O errors. Since classical software finds out about the disk size by looking at the LBA capacity field of the Identify information, such software will not suspect that the disk actually is larger.

The actual total size of the disk is read using the READ NATIVE MAX ADDRESS command. This "soft disk size" is set using the SET MAX ADDRESS command. It comes in two flavours: if the "volatile" bit is set, the command will have effect until the next reboot or hardware reset; otherwise the effect is permanent. It is possible to protect settings with a password. (For details, see the ATA standard.)

This clipped size has (at least) two applications: on the one hand it is possible to fake a smaller disk, so that the BIOS will not have problems, and have Linux, or (for DOS/Windows) a disk manager restore total size; on the other hand one can have a vendor area at the end, inaccessible to the ordinary user.

For many of the disks discussed above, setting a jumper has precisely this effect: LBA capacity is diminished while the native max capacity remains the same, and the SET MAX ADDRESS will restore full capacity.


The CONFIG_IDEDISK_STROKE option of Linux 2.4.19/2.5.3 and later, will tell Linux to read the native max capacity and do a SET MAX ADDRESS to get access to full capacity. This configuration option lives under the heading "Auto-Geometry Resizing support" in the "IDE, ATA and ATAPI block devices" kernel configuration section.

The configuration option went away in 2.6.7 and was replaced by a (per-disk) boot parameter, so that one can say "hda=stroke".

With this "stroke" option jumpered disks will in many cases be handled correctly, i.e., be seen with full capacity (in spite of the jumper). And the same holds when the disk got a Host Protected Area in some other (non-jumper) way.

This is the preferred way to handle disks that need a jumper because of a broken BIOS.

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