The USB port power limit dated as far back as the iBook G3/TiPB, the ports share power with the 56k modem, bluetooth, light sensor(15/17" Alu PB G4) and on late era AluPB/pre-unibody MB/MBP the iSight are on the shared bus. If I remember the iMac G5 iSight model also suffered one port being castrated, Apple listed the port to be only used for the wired keyboard+mouse--this limit ended with the Core 2 Duo(mid-2007 iMac).
Only solution I can recommend is if you must use a HDD/SSD storage device would be:
--Single platter 5400 HDDs such as Seagate Thin(7mm), Hitachi/HGST 7mm or the new WD 7mm series(320GB/500GB are single platters). Don't attempt multi-platter 5400/7200 RPM drives as they will be prone to random disconnects which jacks up SMART error rates, learned this headache with a Spinpoint MG 400GB in a USB enclosure.
--Don't use a fast SSD, Samsung 850 & SanDisk Extreme SSDs require too much juice... lower tier SanDisk Plus/Z-series and others are less power demanding.
If you must buy a pre-fab external HDD, Seagate Backup Plus Slim 1TB uses the Seagate Thin 7mm HDD and the ultra-compact Seagate Seven(500GB Seagate Thin) are power friendly. Don't bother with WD Passports past 500GB, the 1TB is supposed to use less power but if you do intensive disk writes(ex: backups, disk images, restoring files) it'll randomly disconnect which leads to corruption--WD used to include a Y-adapter with Costco & BJs specific models but that ended and you're stuck buying one from NewEgg or Amazon.
