seek on raw magtape

[John Cornelius]

The reason you cannot randomly access tapes for writing is because if an error
occurs while writing, the procedure is to erase 3 inches of tape. Assume that
you had written 3 records of directory. If, while trying to rewrite the middle
one, an error occurs the system will back up over it, erase 3 inches of tape,
and attempt to rewrite the record. This would clobber the 3rd record and
perhaps several others as well since the standard gap between records is only
1/2".

A related issue is the location of the erase head on a tape drive. When writing
tape the drive erases tape ahead of the writing head. The erase head can be as
much as an inch ahead of the write head. This too can clobber the next record.

John Cornelius
Western Scientific