Can I use external storage disk via usb port to farm? What is the difference and disadvantages?
If you are talking about the transfer speed of the data via USB2 you shouldn't have a problem unless you have a very high speed internet connection.
Usually data transfers are as fast as our home internet bandwidht, thus you may use also a laptop disk to farm.
But you should leave your system on day and night, if you close your computer every night you will loose the contract of the stored data in your disk, thus no storj for you
May there is the possiblility that in the further development a drivespacehoster is able to offer his space only at a current timeframe of a day? That meens during a fixed frame there will be no heartneat check and he does not loose his contracts. So his PC dont have to be running all night long. Probably the customer dont need access to his data all night long?
This would not be very practical, since your "customers", i.e. the users that have shards of their data stored on your harddrive, are likely living in a completely different timezone, so what for you will be nighttime, for at least some of the users will be daytime, and they are going to demand access to their data 24/7. So if you want to shut down your computer for any significant period of time, you need to expect to lose your contracts.
In any case, if this feature would be allowed on the Storj network, and you offered limited time access contracts, I doubt many users would take you up on the offer, unless perhaps your prices were significantly lower than what other farmers offer.
