I can say only in general cos i have no experience with Mac (we don't use them here). Ext drive for desktop - 500 would be good (i don't know your prices but here it's good balance between quantity of Gigs and money.) I prefer Seagate - very good reputation (but i use Windows).
Portable sounds good but i think it's all depend on what you need to use it - do you need only usb ports or external power source is acceptable. Cos Hard drives with ext PC costs less money (but they have bigger size too).
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Ext drive for desktop - 500 would be good (i don't know your prices but here it's good balance between quantity of Gigs and money.) I prefer Seagate - very good reputation (but i use Windows).
Portable sounds good but i think it's all depend on what you need to use it - do you need only usb ports or external power source is acceptable. Cos Hard drives with ext PC costs less money (but they have bigger size too).