No 256 heads option in Menu Geometry? Cylinder boundaries and manually changing partition information.

How to use TestDisk to recover lost partition
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Posts: 4
Joined: 23 Mar 2018, 00:22

No 256 heads option in Menu Geometry? Cylinder boundaries and manually changing partition information.

#1 Post by zxcvb » 07 Apr 2018, 21:55

Testdisk's Menu Geometry says "Enter the number of heads" "The most common CHS head values are: 255, 240 and sometimes 16." (I'm using Testdisk 2006 version 6.3 (Windows) which is compatible with Windows NT 4.0.)

However my system uses 256 - CHS 1024/256/63 (sector size=512).

Testdisk will not let me input 256 heads.

I know about the DOS/Windows 95 bug that causes systems to hang if the hard drive is over 4gb and the CHS settings use 16383*16*63. BIOS's Large mode translates 16 into 256 (255=FF).

Is it absolutely necessary for partitions to start on cylinder boundaries?

From what little I've found about this subject online, it appears that the answer is no. Worst case scenario, the user ends up with wasted sectors between partitions. According to one guy, "Cylinder/head/sector (CHS) values have passed through several stages, from an accurate representation to a convenient fiction to an inconvenient fiction to worse than useless to abandoned."

I ask because a non-OS hard drive has one partition set at 256 heads, and another partition set at 240. There is no data corruption - Windows doesn't use CHS, although Dr Dos 7.03 does and has choked on the inconsistency and caused minor hassles.

Reformatting is not an option right now.

I suppose I should manually re-edit one of the partition's figures so both use the same number of heads. If I change the 240-head partition to 256 heads, then I will get a new CHS figure like, for example,

partition begins at 160-510-1
partition ends at 20-1020-63

But before I do that any potential risks I should know about? Other than "make sure you've got your numbers right."

Thanks to one and all who can help.

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