How fast is a disk when used in a USB enclosure? I tested USB by adding a fast disk to a USB enclosure and ran a standard benchmark.
The best way to test a USB enclosure is to use a disk that is faster than USB. I used an SSD, Solid State Disk, and one of the fastest SSDs. The following results show consistent speeds around 30 MegaBytes per second.
Minimum read rate: 31.4 MB/s
Maximum read rate: 35.6 MB/s
Average read rate: 33.0 MB/s
Minimum write rate: 26.9 MB/s
Maximum write rate: 33.9 MB/s
Average write rate: 32.7 MB/s
Average access time: 0.6 ms
The access time is almost irrelevant. Access time is usually disk related. Magnetic disks have high access times. SSDs should have low access times. USB adds a constant overhead. Your operating system and file system add additional overheads depending on the access type. If you are replacing a row in a database, there is an overhead from the operating system, from the file system, and the database. If your access is the start of a new file, the disk access time will be buried in the massive file open overheads from the operating system and file system.
Try two file copies from Windows XP to the USB disk. First test one large file, perhaps one GigaByte. Then find lots of little files that add up to one GB and copy them to the USB disk. Copying lots of little files will take longer, perhaps several minutes longer, due to all the file open overheads. Repeat the test using Linux instead of Windows XP. Copying the one large file will take the same time. Copying all the little files will will be faster than XP but still slower than copying one long file.
Connected to a netbook
I copied 10 GB of photographs to the USB drive then to my netbook. The mixture contained medium size files and small files. The copy to the the netbook averaged 17.7 MB/s. The small files contributed to the slowdown plus the netbook processing could not keep up with the maximum USB speed because the netbook disk is too slow. Every file copy required a file creation on the netbook disk with all the overheads from the operating system and file system.
Could I reduce the file system overheads? I could copy the files from the netbook to the USB disk and see if there is a difference.
I formatted the USB disk then ran the disk benchmark in Linux. The results were similar to the previous benchmark and indicate the netbook can process the USB disk at the same speed as a desktop computer. Some results varied by up to 1.0 MB/s and that is related to the accuracy of benchmarks. You can create artificial benchmarks with less variation in speed measurements between tests but that type of benchmark is difficult to repeat across many computers.
I then copied the pictures to the disk and the speed increased to 19.9 MB/s. Most of the overhead difference, 2.2 MB/s, is caused by the netbook disk slowing down file creation when copying to the netbook and part is just the slight random variations between tests.
Maximum theoretical speed
USB operates at 480 Megabits per second, 480 Mb/s. You could convert 480 Mb/s to 60 MegaBytes per second, 60 MB/s, by dividing the bit rate by 8, the number of bits in a byte. Most transfer protocols include check bits and other overheads of about 2 bits per byte. Allowing for those overheads, 480 Mb/s becomes 48 MB/s.
The USB protocol appears to add another overhead of 10%, reducing 48 MB/s to 43.7 MB/s. I have measured a USB speed of 42 MB/s and it is rare. The maximum read rate of 35.7 MB/s in the benchmark is good for an avarage mix of files and should be close to 40 MB/s for one large file.
The USB disk with the SSD inside is fast enough for transfers small enough to fit on my netbook. 30 MB per second is one GB in 33 seconds or about 2 GB per minute. My snaps from a typical walk in the bush reach 8 GB because I used an 8 GB card in my camera and frequently filled it. Now I use a 16 GB card. The Sandisk Extreme cards read at 30 MB/s and USB is fine for that type of work.
When I backup a computer before an upgrade, I need to copy hundreds of GB and USB 2.0 is too slow. eSATA is the alternative. My USB disk has an eSATA connection but most of my computers do not have eSATA. USB 3.0 is the next version of USB and is faster than eSATA. Most new computers have USB 3.0 and some have dropped eSATA.
I will set up a large disk, perhaps 1 TeraByte, in a case with USB 3.0 and eSATA for those upgrade backups.