According to several manufacturers, mounting a 3/5" hard drive horizontally, vertically, or sideways doesn't affect the hard drive life significantly.
These are statements taken from the hard drive literature at each manufacturer's website; it's four years old but things probably haven't changed much.
The drive will operate in all axes (6
directions). Performance and error
rate will stay within specification
limits if the drive is operated in the
other orientations from which it was
Physical mounting of the drive: WD
drives will function normally whether
they are mounted sideways or upside
down (any X, Y, Z orientation).
The hard drive can be mounted in any orientation.
As long as it is securely attached to
the chassis, hard disk drives may be
mounted either horizontally or
vertically depending on how your
computer's case is constructed.
When asked if the drive could be mounted at askew angles, their official positions were:
Manufacturer Contact method Response
------- --------------------- ---------------------
WD Tech support, email 90 degrees.
Hitachi Hitachi documentation 90 degrees.
Samsung Tech support, phone 90 degrees.
Fujitsu Tech support, chat 90 degrees +-5.
Seagate Tech support, email 90 degrees preferred,
but diagonal OK.
Maxtor Tech support, phone 90 degrees preferred, but in
real world, whatever.
By 90 degrees, they mean vertical, horizontal, or sideways.