Quark + unidisk + apple //c problem

[bchurch@oucsace.cs.ohiou.edu (Robert Church)]

I saw an ad for a Quark QC 20 meg hard drive. I'd like to buy it for my //c
but the current owner says that he can't get it to work with the //c and
a unidisk 3.5. He says it's fine alone but goes nuts if you connect the 3.5.
Does anyone know anyway around this problem? I'd really like to add a harddrive
to my //c but can't do without my unidisk 3.5. 

Thanks
Bob Church
bchurch@oucsace.cs.ohiou.edu