Servo as a spindle motor replacement - what to look out for?

[Voicecoil]

No, generally a single ended driver just sinks current. When you turn it off, the current into the driver stops. If there is a decent capacitance in / around the opto LED, it takes a few hundred nanoseconds to actually turn off completely.

Depends whether it's a bipolar/TTL or CMOS driver. The former tend to be MUCH better at sinking than sourcing, whilst the latter are pretty symmetrical due to the complementary output stage, e.g. the datasheet for the 74HC245 which is quite popular on cheapo BOBs shows 0.18V droop when sourcing 6mA and 0.15V when sinking 6mA: 30mV difference in the ~3V across the series resistor. Differential would still be a bit better as you potentially have 2x the voltage discharging aforementioned capacitance, but it's a darn sight better tah open collector drive.