Radio Shack TRS-80 Model II Reference Manual page 118

MODEL II TRSDOS
Using BACKUP To Recover Data (Alternate Directory
Required).
Of all tracks on a diskette, the directory is used the most, therefore it tends to wear
out first. When a primary becomes unreadable,
TRSDOS
cannot perform normal
disk
I/O
using that diskette. If an alternate directory is available,
TRSDOS
will use it
automatically. This will slow down disk
I/O.
If you begin having diskette errors with a particular diskette, use
BACKUP
with a
destination diskette which is known to be good (no tlaws). During the backup
process,
TRSDOS
will use the alternate directory, if one is available and is needed, to
recreate a good directory on the destination diskette. (If the destination has both
primary and alternate directories, both will be contain the same good information.)
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