There are two main types of Acer Aspire one ZG5.
The first one is AAO110, which is the SSD model. The original SSD's are notorious for their poor speed, specifically writing speed is very poor. The AAO110 can use 1,3" and 1,8" ZIF PATA drives, e.g. some models used in Apple Ipods fit in as long as they are physically of right size. If the drive-to-be-installed is physically too big, you could consider removing gently some plastics of the netbook's case but beware - don't remove too much! Or in case it is a SSD, try to figure out how to remove possible casing (probably voiding the guarantee) and use the "bare" drive.
The other main type is AAO150, which can use normal 2,5" SATA laptop hard drives and SSD's. For the AAO150 it is a lot easier to find suitable drives, they are practically everywhere.