$ perl - 
#!/usr/bin/perl -l --
$_ = q{Disk - HD 600GB 15K};
print / (\d+(?:\.\d+)?)GB\W/;
__END__
600

$ perl -Mre=debug - 
#!/usr/bin/perl -l --
$_ = q{Disk - HD 600GB 15K};
print / (\d+(?:\.\d+)?)GB\W/;
Compiling REx " (\d+(?:\.\d+)?)GB\W"
Final program:
   1: EXACT < > (3)
   3: OPEN1 (5)
   5:   PLUS (7)
   6:     DIGIT (0)
   7:   CURLYX[0] {0,1} (14)
   9:     EXACT <.> (11)
  11:     PLUS (13)
  12:       DIGIT (0)
  13:   WHILEM (0)
  14:   NOTHING (15)
  15: CLOSE1 (17)
  17: EXACT <GB> (19)
  19: NALNUM (20)
  20: END (0)
anchored " " at 0 floating "GB" at 2..2147483647 (checking floating) minlen 5
__END__
Guessing start of match in sv for REx " (\d+(?:\.\d+)?)GB\W" against "Disk - HD 600GB 15K"
Found floating substr "GB" at offset 13...
Found anchored substr " " at offset 4...
Starting position does not contradict /^/m...
Guessed: match at offset 4
Matching REx " (\d+(?:\.\d+)?)GB\W" against " - HD 600GB 15K"
   4 <Disk> < - HD 600G>     |  1:EXACT < >(3)
   5 <Disk > <- HD 600GB>    |  3:OPEN1(5)
   5 <Disk > <- HD 600GB>    |  5:PLUS(7)
                                  DIGIT can match 0 times out of 2147483647...
                                  failed...
   6 <isk -> < HD 600GB >    |  1:EXACT < >(3)
   7 <sk - > <HD 600GB 1>    |  3:OPEN1(5)
   7 <sk - > <HD 600GB 1>    |  5:PLUS(7)
                                  DIGIT can match 0 times out of 2147483647...
                                  failed...
   9 < - HD> < 600GB 15K>    |  1:EXACT < >(3)
  10 <- HD > <600GB 15K>     |  3:OPEN1(5)
  10 <- HD > <600GB 15K>     |  5:PLUS(7)
                                  DIGIT can match 3 times out of 2147483647...
  13 <HD 600> <GB 15K>       |  7:  CURLYX[0] {0,1}(14)
  13 <HD 600> <GB 15K>       | 13:    WHILEM(0)
                                      whilem: matched 0 out of 0..1
  13 <HD 600> <GB 15K>       |  9:      EXACT <.>(11)
                                        failed...
                                      whilem: failed, trying continuation...
  13 <HD 600> <GB 15K>       | 14:      NOTHING(15)
  13 <HD 600> <GB 15K>       | 15:      CLOSE1(17)
  13 <HD 600> <GB 15K>       | 17:      EXACT <GB>(19)
  15 <HD 600GB> < 15K>       | 19:      NALNUM(20)
  16 <HD 600GB > <15K>       | 20:      END(0)
Match successful!
600
Freeing REx: " (\d+(?:\.\d+)?)GB\W"