This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
Due to an obscure interaction between Acorn's IDE implementation and the Western Digital drives a problem occurs in that the computer becomes unable to find the Boot record that details the shape, format and other data of the hard drive. This results in a somewhat alarming and frustrating series of error messages that seem to indicate that the drive isn't formatted at all. (And by extension implying that you have lost all your information stored on the drive.)
Fortunately that isn't the case. In reality your information is safe and sound on the drive and you merely need to give ADFS a helping hand in finding the boot record, after which it can carry on as normal. This BASIC program supplies dummy values to ADFS that allow it to do that.
REM> Specify drive%=4 DIM rec% 64 FOR I%=0 TO 60 STEP 4:rec%!I%=0:NEXT rec%?00= 9 : REM Sector size, 2^9 = 512 rec%?01= 8 : REM Sectors per track rec%?02= 1 : REM Heads rec%!16=4096 : REM Disc size in bytes rec%?41= 0 : REM LBA mode disabled SYS "ADFS_SectorDiscOp",,15+(rec%<<6),drive%<<29 END
(Thanks to Eduard Pfarr for this program.)
Running this program once, during boot up, will allow the drive to be used normally. This does mean, for the moment, that you cannot boot from a Western Digital IDE drive.
NB for RISC OS 3.10, or earlier versions of RISC OS, you will need to replace the ADFS_SectorDiscOp with an ADFS_DiscOp call instead.