According to our not-so-secure information, in order to install ROSA most users download ISO images from our sites and then either install from this ISO directly or burn it to a USB flash or DVD disk (yes, it looks like DVD devices are still in use nowadays). Unfortunately, despite the common tendency of improving Internet speed and quality, it can still happen that something goes wrong during image download and user gets a broken file. Installation from such a file will be likely unsuccessful, so it is a good idea to preliminary check ISO file correctness.
A traditional way of verifying file content is to calculate its hash some and compare it to the expected value which is usually placed in a separate file (with .md5 or .sha1 or some other extension, depending on algorithm used to calculate the sum) near the ISO image. So besides the ISO file itself, you should download a file with its hash sum, calculate hash of the downloaded ISO (by running md5sum ROSA.iso) and compare the result with the expected one. Some programs will do some of these tasks for you automatically - e.g., K3b used to burn ISO images can automatically calculate hash sum of the image and compare it with the one provided in .md5 file (if the latter is found near the ISO file).
A less known way to verify ISO image content is to embed its hash sum into the file itself. The thing is that ISO9660 files contain an unused section whose size is large enough to store MD5 sum. To calculate MD5 sum of the ISO image content and embed it in the file itself, one can use implantisomd5 tool which is a part of isomd5sum package. To verify the image content (calculate its sum and compare it with the embedded value) one can use checkisomd5 tool from the same package.
All modern ROSA Desktop Fresh images contain MD5 sum embedded by means of implantsiomd5, so to verify downloaded ISO image you can simply launch
$ checkisomd5 ROSA.iso
This command has a couple of additional options: "--verbose" will provide you with additional inmformation about verification process, "--gauge" will display progress by means of numbers from 0 to 100 and "--md5sumonly" will force the tool to calculate and print hash sum of the ISO content, but not to compare it with embedded value.
You can launch checkisomd5 not only against ISO files, but also against block devices - for example, if you have a DVD with ROSA in your DVD drive, you can verify its content by means of checkisomd5 /dev/dvdrw.
Please not forget to verify downloaded images. This doesn't take long and can save you a lot of time in future if you try to install a system from a broken image.